(1) Field of the Invention
The present invention relates to a method of preparing a naphthol-modified phenolic resin which is a suitable curing agent for epoxy resins, and to an epoxy resin molding material for sealing electronic parts which contains the naphthol-modified phenolic resin prepared by the method.
The epoxy resin molding material for sealing electronic parts of the present invention is suited to production of VLSIs (very large scale integrated circuits) which require both a high glass transition temperature and an excellent resistance to reflow cracking.
(2) Description of the Related Art
Epoxy resin molding materials have been used widely to seal electronic parts, such as transistors and ICs. The reason is that epoxy resins are well balanced in properties including electrical properties, moisture resistance, heat resistance, mechanical properties and adhesive properties to inserted parts. In particular, epoxy resin molding materials composed of o-cresol novolac epoxy resins and phenol novolac curing agents are particularly well balanced in these properties and are now mainly used as the base resins of molding materials for sealing ICs.
ICs have been thinned and downsized for high density mounting on printed wiring boards. Thinned and downsized packages however are apt to crack during mounting by soldering (fusing step). This is the problem pertaining to ICs for surface mounting that is to be solved at present. One means proposed for solving this problem is molding such thin packages, such as TSOPs (thin small outline packages), by using a sealing material containing a biphenyl skeleton epoxy resin Japanese Patent Application Kokai Koho (Laid-open) No. 58-39677!. Biphenyl skeleton epoxy resins have low hygroscopicity and high moisture resistance, and sealing materials containing the epoxy resins exhibit excellent resistance to fellow cracking.
However, when cured with phenol novolac resins, the biphenyl skeleton epoxy resins suffer a decrease in glass transition temperature. To solve this problem, it has been tried to modify phenol novolac resins by introduction of various substituents or naphthol skeletons Japanese Patent Application Kokai Koho (Laid-open) No. 4-199856!.
However, introduction of very bulky substituents or rigid skeleton structures, such as naphthol skeletons, into curing agents adversely affects the softening point and flowability of molding materials for sealing ICs, and has not yet been put into practical application.